My router was built with rails, bearings and balls crews from linearmotionbearings. The cutting area is 500 x 600mm and the whole build came in at about £1000. I did what Jonathan has suggested and built it from mild steel box section.

Here is a link to the build thread, it may give you some ideas http://www.mycncuk.com/forums/showth...-framed-router

Good luck with the build.